Because a polymer-based composite material comprising carbon fibers and a matrix resin is light-weight and has excellent mechanical properties, it is broadly used for sporting goods uses, aerospace uses and general industrial uses. Although various methods are employed for production of carbon fiber-reinforced composite materials, a method for using a prepreg, which is an intermediate substrate prepared by impregnating a non-cured matrix resin into reinforcing fibers, is broadly applied. In this method, usually, a formed product of a composite material can be obtained by stacking prepregs and thereafter heating them.
As a matrix resin used for prepregs, thermoplastic resins and thermosetting resins are both used, and resins such as epoxy resins, maleimide resins, cyanate resins and polyimide resins are used.
With respect to mechanical properties of carbon fiber-reinforced composite materials, although the tensile strength thereof has been greatly increased as the tensile strength of carbon fibers increases, increase of the compression strength thereof is small even if high tensile-strength fibers are used instead of standard tensile-strength fibers. Accordingly, flexural strength important for practical uses, which is determined depending upon a smaller strength of either tensile strength or compression strength, is to be determined by the compression strength. Therefore, the compression strength is very important for uses of structural materials on which compression or flexural stress is applied. Particularly, the compression strength is an extremely important property for use as a primary structure material. Further, in a case of an aircraft, since there are many bolt holes, an open-hole compression strength becomes important.
Further, because mechanical property, particularly the compression strength, greatly decreases under a hot-wet condition, an open-hole compression strength under a hot-wet condition, which is a property under a severe condition, becomes a very important index. The open-hole compression strength (referred to as "Open Hole Compression": OHC) is a property determined as a compression strength of a connecting portion, that is, a compression strength at a position corresponding to a portion with a hole for connection, by small-scale measurement.
Further, when used as a primary structure material, a residual compression strength after impact damage due to hailstone or tools also becomes important. Therefore, a compression strength after impact (referred to as "Compression After Impact": CAI), which is determined by small-scale measurement as a residual compression strength after impact damage due to foreign materials such as collision of stones or fall of tools, is one of important properties inevitable for damage tolerance design.
Although a conventional polymer-based composite material has an advantage of light-weight, the above-described open-hole compression strength under a hot-wet condition and compression strength after impact thereof are not still sufficient, and these properties are desired to be further improved for enlarging the range of the applicable uses.
In order to increase the open-hole compression strength, it is known that it is effective to increase the amount of fibers taking charge of load or to increase the stiffness of a resin, but these methods are limited to a certain level. In order to increase the compression strength after impact, it is known that it is effective to increase the amount of a resin or to use a highly-tough resin such as a thermoplastic resin. However, the method for increasing the open-hole compression strength and the method for increasing the compression strength after impact are contrary to each other, and both methods are in a trade-off relationship. Therefore, in a conventional technology, an excellent carbon fiber-reinforced composite material such as one having an open-hole compression strength under a hot-wet condition and a compression strength after impact under a room-temperature condition of each 275 MPa or more has never been obtained.
With respect to noncircular cross-section carbon fibers, many technologies in pitch-based carbon fibers are disclosed. When melt spinning is carried out using pitch as a raw material, a nonuniform crystal structure is created in green fibers by a shear stress during spinning, this is left as a nonuniform crystal structure in carbon fibers after carbonization and is observed as a lamella structure, and only carbon fibers poor in strength and elastic modulus can be obtained.
For example, in technologies disclosed in JP-A-SHO 61-6313, JP-A-SHO 62-117821, JP-A-SHO 62-231024 and JP-A-SHO 62-131034, lamella structures are observed in carbon fibers, and the mechanical properties thereof are low. In particular, pitch-based carbon fibers are low in compression strength as compared with polyacrylonitrile-based carbon fibers and cannot indicate properties which can be applied to a primary structure material.
This presence of a lamella structure can be easily observed as a broken texture extending radially from a center of a broken surface of a carbon fiber toward outside when a transverse cross section obtained by cutting a carbon fiber is observed using a scanning electron microscope (SEM) as described in, for example, JP-A-SHO 61-6313 and JP-A-SHO 62-131034. FIG. 3 is a schematic view of a lamella structure. The lamella structure means a leaf-like orientation structure "a" extending radially in the cross section of a carbon fiber "F" shown in FIG. 3.
As to noncircular cross-section carbon fibers of polyacrylonitrile-based carbon fibers, it is described in a prescript of the twentieth International SAMPE Technical Conference that they can be obtained by melt spinning. In polyacrylonitrile-based carbon fibers, a lamella structure such as one recognized in pitch-based carbon fibers is not observed. However, in order to carry out melt spinning, a large amount of a plasticizer must be added or the molecular weight of a polyacrylonitrile polymer must be decreased. Therefore, the orientation of a precursor is low, and carbon fibers obtained by carbonizing the precursor become poor in mechanical properties. Particularly, in a case where a plasticizer is added at a large amount, the affect is remarkable.
A technology for providing a noncircular cross-section to polyacrylonitrile-based carbon fibers by melt spinning is disclosed in U.S. Pat. No. 5,227,237, and therein it is described that the technology is effective to increase a compression strength of a composite material. However, with respect to combination with a matrix resin, there is only a description that a general thermosetting or thermoplastic resin can be used, and there is no suggestion on a synergetic effect with a resin such as one according to the present invention. In the examples thereof, merely a general epoxy resin is used. Further, with respect to surface treatment after carbonization, there is only a description that it is preferred to apply electrolysis in a sulfuric or nitric solution or oxidation in a gas or liquid phase, there is no particular contrivance, and there is no description as to amount of functional groups on the surface.
According to the investigation by the inventors of the present invention, it has been found that, if the amount of functional groups on the surface is not controlled to a adequate level by contriving surface treatment even in a case of noncircular cross-section carbon fibers, 90.degree. tensile strength which is a typical index of a bonding strength between fibers and a resin is low and cracks due to debonding are likely to occur at an interface between the fibers and the resin. Particularly, in a case where a carbon fiber-reinforced composite material is used as an aircraft primary structure material, such cracks become very serious. Namely, if cracks are generated, water enters into the cracks, the cracks are enlarged by freezing of the water at a low temperature, and the breakage is accelerated. Therefore, it is very important to suppress the generation of cracks.
With respect to surface treatment of carbon fibers, it is well known that it is effective to introduce an oxygen-containing functional group into the surface of carbon fibers for improving a wettability with a matrix resin. It is disclosed in, for example, JP-A-HEI-4-361619, that it is effective to control a surface oxygen concentration of carbon fibers to a specified level. Further, it is disclosed in, for example, JP-B-HEI-4-44016, JP-A-HEI-2-210059, JP-A-HEI-2-169763, JP-A-SHO 63-85167 and JP-A-SHO 62-276075 that it is effective to specify not only surface oxygen concentration but also surface nitrogen concentration for improving bonding property with a matrix resin. Furthermore, it is disclosed in U.S. Pat. No. 5,462,799 that it is effective to control not only surface oxygen concentration or surface nitrogen concentration but also surface hydroxylic concentration and surface carboxylic concentration to specified ranges for improving bonding strength with a resin. However, these technologies are all on circular cross-section carbon fibers, and there is no disclosure with respect to control of amount of functional groups on surface due to surface treatment and contrivance of the surface treatment on noncircular cross-section carbon fibers. Although it has been considered that noncircular cross-section carbon fibers have a high adhesion strength with a resin by so-called anchor effect indicated mechanically by the shape of the cross section, it is not always correct.
On the other hand, in order to increase a compression strength under a hot-wet condition by a technology on a matrix resin, increase of an elastic modulus of the resin is effective, and particularly, it is important to suppress reduction of an elastic modulus under a hot-wet condition. Increase of crosslinking degree of an epoxy resin has been proposed for increasing the elastic modulus of the resin, and reduction of water absorption and introduction of a thermal-resistant structure have been proposed for suppressing reduction of an elastic modulus under a hot-wet hydroscopic condition.
As a resin composition for prepregs balanced in impact resistance, thermal resistance and water resistance, a resin composition blended with an epoxy resin whose main constituent is an epoxy resin having a triglycidylaminophenol structure, diaminophenylsulfone and polyethersulfone or polyetherimide is disclosed in JP-A-SHO 62-297316 and JP-A-SHO 62-297312. However, the epoxy resin having a triglycidylaminophenol structure has a problem that properties under a hot-wet condition greatly decrease as compared with properties under a room-temperature dry condition.
Further, in order to suppress reduction of properties of a cured epoxy resin due to water absorption, a particular diamine curing agent which can reduce water absorption and an epoxy resin blended therewith are disclosed in JP-A-SHO 59-21531 and JP-A-SHO 60-67526. However, when such a particular diamine curing agent is used, it must be added at a large amount, and therefore, the viscosity of the resin becomes too high, and there are problems that the design of the resin is remarkably limited as well as blend of a thermoplastic resin for improving impact resistance, which usually causes a further increase of the viscosity, or layer interface reinforcing technology cannot be applied.
As a resin composition excellent in thermal resistance and flowability during curing process, a blend of 3,3'-diaminodiphenylsulfone and a thermoplastic resin having a glass transition temperature of 100.degree. C. or higher to an epoxy resin is described in JP-B-HEI-7-78138, and therein it is described that it is preferred to blend tetraglycidyldiaminodiphenylmethane at a content of 50 to 80% relative to the whole of the epoxy resin particularly in a case of attaching importance to thermal resistance. However, a cured material of such a resin composition is poor in resin toughness and impact resistance.
As a prepreg excellent in impact resistance, so-called particulate inter-layer toughening technology for adding thermoplastic resin particles to a base resin of a thermosetting resin is disclosed in JP-B-HEI-6-94515. In this, a composition using tetraglycidyldiaminodiphenylmethane at a content of 90% relative to an epoxy resin and 4,4'-diaminodiphenylsulfone at a mole of 0.175 time the mole of the epoxy resin for a base resin of a thermosetting resin and polyethersulfone at a content of 10% is disclosed. However, in such a composition, even if it is effective for increasing compression strength after impact, it is not effective for increasing open-hole compression strength.
Further, another inter-layer toughening technology is disclosed in JP-A-HEI-5-1159 and JP-A-HEI-4-268361, and these disclosures describe examples wherein an epoxy resin prepared from tetraglycidyldiaminodiphenylmethane and triglycidylaminophenol as a base resin, 3,3'-diaminodiphenylsulfone as a curing agent and polysulfone or polyethersulfone oligomer having an amine end group as a thermoplastic resin are used. However, in such a resin composition, even if it is effective for increasing impact resistance, because triglycidylaminophenol is present at a content of 40 to 50% in the epoxy resin, the elastic modulus of the resin under a hot-wet condition is not high, and therefore, the compression strength under a hot-wet condition is insufficient.